Mother left fat on duck (7)
I believe the answer is:
mallard
'duck' is the definition.
(mallard is a kind of duck)
'mother left fat on' is the wordplay.
'mother' becomes 'ma' (ma is a kind of mother).
'left' becomes 'l' (common abbreviation).
'fat' becomes 'lard' (I've seen this before).
'on' says to put letters next to each other.
'ma'+'l'+'lard'='MALLARD'
(Other definitions for mallard that I've seen before include "web-footed bird" , "The famous steam train" , "Engine holding the world speed record for a steam locomotive, set at 123mph in 1938" , "Type of duck - world speed record holding steam engine (1938)" , "Common wild duck" .)